Directional solidification processes and apparatus are widely used in the metal casting art for producing a polycrystalline columnar microstructure or a single crystal microstructure in metal castings and resultant advantageous mechanical properties in certain directions of the casting. Directional solidification processes/apparatus are widely used in the aerospace industry to manufacture components, such as hot section blades and vanes for gas turbine engines, where the components must operate under conditions of high temperature and high stress.
U.S. Pat. No. 3,931,847 describes a directional solidification system of the withdrawal type wherein a melt filled, heated ceramic investment mold disposed on a chill plate is withdrawn from a heating chamber past a radiation (heat control) baffle to maintain a steep thermal gradient in the melt in the mold needed to achieve a solidification front that moves upwardly through the melt as the mold is withdrawn so as to form the desired unidirectionally solidified microstructure. The investment mold is provided with an auxiliary thermocouple-receiving post cavity for monitoring the temperature inside the mold during mold preheating prior to filling with melt. The thermocouple received in the post cavity is mounted on the chill plate so as to remain fixed in position in the post cavity for movement with the mold. A second thermocouple extends through an opening in the mold base for monitoring temperature outside the mold casting cavity in the heating chamber. The second thermocouple also is mounted on the chill plate for movement with the mold. A third thermocouple is mounted on the heating chamber cover so as not to move with the mold and functions as a temperature sensor for a heater process control. The arrangement of thermocouples described in the patent is provided to improve control of the mold temperature/chamber temperature during preheating and the solidification front in the melt during the directional solidification process.
U.S. Pat. No. 5,197,847 describes apparatus for manufacturing directionally solidified columnar or single crystal castings wherein mold withdrawal speed relative to a heating chamber is controlled during a casting trial of a particular cast component in response to melt temperature signals from thermocouples positioned in the melt in the mold. The mold withdrawal speed is controlled to maintain the solidification front at a more or less fixed location relative to a heat insulation (heat control) baffle. A computer process controller is programmed using the actual casting trial data (e.g. withdrawal speed profile data) to control mold withdrawal speed and other casting parameters during subsequent repeat casting cycles to produce like cast components without thermocouple contact with the melt.
U.S. Pat. No. 4,964,453 discloses directional solidification of superalloys by imposition of a predetermined temperature profile across the solidification front in the melt in the mold. The temperature profile across the solidification front is determined by thermocouples contacting the melt.
U.S. Pat. No. 4,281,985 describes a mechanical arrangement for automatically positioning thermocouples relative to a crucible in a vacuum furnace to monitor temperature of a charge melted in the crucible. A bellows is subjected to differential pressure so as to move between retracted and expanded positions for locating the thermocouples relative to the charge in the crucible.